Relief valve and flare action items: What plant engineers should know

This article can help the plant engineer review the concerns developed by the design engineer. Implementing field modifications without performing such a review is costly and exposes a facility to unjustified risks.

When most companies implement the process safety management
standard, they routinely or periodically review the relief
systems and flare systems design bases to ensure compliance
with corporate, industry and/or government standards, hereafter
referred to as Recognized And Generally Accepted Good
Engineering Practices (RAGAGEP). To mitigate concerns before
implementing projects, it is advisable for plant
engineers to consider several items when reviewing a concerns
list:

Relief systems review methodology

Relief systems review priorities

The process designers familiarity with the process
and/or plant when concerns are being reviewed

The plant engineers understanding of the
differences between compliance and best-in-class
practices.

An article published in 2000 concluded that up to 40% of the
installations evaluated had unidentified concerns.1
Since the publication of this article, many of these concerns
have undergone a more detailed review showing that
modifications to the facility were not required to address
these concerns.

The purpose of this article is to help the plant engineer
review the concerns developed by the design engineer.
Implementing field modifications without performing such a
review is costly and exposes a facility to unjustified
risks.

For the purposes of clarity, the following terms used
throughout this article are defined as such:

Plant engineerThe facility or
owners engineer who is responsible for reviewing the
concerns and determining if facility modifications should be
implemented

Process designerThe individual who
is responsible for analyzing the relief device and
overpressure protection system and developing the
concerns

ConcernsItems that are listed
(prior to being fully reviewed and accepted) as deviations
from industry or company standards.

At the end of the relief systems design basis project, the
process designer typically identifies many concerns. As most facilities want to comply with
RAGAGEP, there is a mandate to resolve these concerns, and
their resolution can be costly.

Generally, most facilities seek to comply with regulations
for existing facilities and to potentially build new equipment
and facilities to a higher standard. This article includes
examples of how to review existing systems to determine if
concerns justify field modifications.

RELIEF SYSTEMS REVIEW METHODOLOGY

Relief systems design basis reviews are typically performed
by contractors that assist in developing project guidelines and
then collect the necessary information. After these initial
actions, the contractors analyze the systems design basis
per the project guidelines and present a list of identified
concerns to the plants engineers and management. Prior to
spending money to upgrade the relief systems, a plant engineer
familiar with the process unit should review the concerns list
to ensure that:

Details of the study are reasonable

Assumptions of the study are reasonable

Facility upgrades, not based on minimum compliance, have
been thoroughly reviewed.

By reviewing the concerns list with these suggestions, a
plant engineer can ensure that costly changes have a basis in
sound engineering and that the expense is justified. Note that
no hierarchical order is implied in this list.

Typically, when a relief systems design basis project is
undertaken, the goal is to produce compliant documentation
efficiently and consistently. To do this, process designers
must base the analysis on a framework to minimize effort and
ensure consistency. This is a practical method for performing a
large-scale relief systems analysis; however, for any
particular concern, the framework may break down and suggest
items that are not actually concerns.

In a recent review project, approximately 40% of the listed
concerns were later found to be acceptable based on a detailed
review. The following sections help walk a plant engineer
through a systematic process and give insight into how to
review the listing of concerns.

Reviewing relief system study details

When the concerns are reviewed from the perspective of the
process designer, the plant engineer can understand how the
framework may have generated potential concerns. Understanding
this process can help the plant engineer identify resolvable
concerns by reviewing the design basis.

Understanding the process. When completing
large-scale relief systems design basis documentation and
design processes, the process designer is usually quite
familiar with relief systems design but may not be familiar
with the particulars of the process or unit. The process
designer, therefore, may make unrealistic judgments about
process upsets. The following are examples of these items:

When process flows can be blocked, or if the normal rate
is possible under upset conditions

Use of the normal/design duty from a reboiler for
relief rate estimation

Equipment that is no longer in service is not properly
protected.

To ensure the best possible analysis, each study should be
reviewed by personnel familiar with the process operation to
confirm that unique process characteristics are captured in the
relief systems documentation.

Credibility of scenario or relief rate. For
each overpressure scenario that generates a concern, the plant
engineer should give particular attention to ensure the
credibility of the scenario or required relief rate. Many
times, an overpressure scenario or the estimated rate may not
be credible. The following are some examples:

Pumps that can only pump to relief pressure if the
upstream system is also upset (however, a simultaneous upset
would be considered double jeopardy)

Systems where overpressure derives from heat input, such
that the relief temperature of the process fluid exceeds the
relief temperature of the utility fluid

Control valve failure calculations that are based on the
capacity of a control valve instead of on another limitation
(e.g., a long section of piping or a pump).

To ensure an accurate analysis, each concern should be
reviewed to verify that consideration has been given to the
determination of the scenario applicability and that the relief
rate estimate is reasonable for the particular process or
unit.

Gathering facility data. The relief systems
analysis process typically limits the amount of places and time
that the process designer can search for process and equipment
data. This limitation is usually defined as a project scope
item and is used to ensure that the project has boundaries.
When reviewing concerns, the plant engineer must ensure that
the process designer has not identified concerns that can be
readily resolved by further searching for process and/or
equipment data. Often, this requires a call to an external
supplier or technical body (e.g., the equipment manufacturer or
national board).

Other execution issues. The relief systems
process typically uses a consistent basis that is often
documented and referred to as site or project guidelines. These
guidelines are beneficial, as they provide a means for
efficient and consistent execution and help ensure that both
the process designer and plant engineer are in agreement on the
details of the analysis. When these generic and prescriptively
conservative guidelines generate concerns, it is imperative
that the team generating the documentation reviews the
fundamentals of the analysis to confirm that the concern is a
legitimate deviation from RAGAGEP and not just a result of the
project execution process.

Reviewing relief system study assumptions

The typical execution method of a project tends to enforce
consistent assumptions. For most of the project, this ensures
that the relief systems design basis is conservative and
compliant with RAGAGEP. To ensure that field modifications are
for items that must be addressed, these assumptions may need to
be challenged when concerns are raised.

Standardization assumptions. Standard and
generally conservative assumptions are specified to ensure
consistency and efficiency. These assumptions help the relief
systems documentation process run efficiently; however, if
generic assumptions result in concerns, they should be
revisited and updated. The following are some examples of these
items:

Liquid levels for equipment

Control valve flow coefficients and trim sizes

Utility pressures (e.g., steam, nitrogen, cooling
water)

Heat exchanger or pump capacities.

To ensure the best possible analysis, the assumptions
associated with each concern should be reviewed and, if
possible, refined to be specific for that system.

Conservative assumptions. The
authors of this article have been carrying out relief systems
analysis for multiple decades and believe that
conservative assumption is frequently used as a
phrase for a simplifying assumption that the process designer
invoked. Furthermore, this phrase typically has nothing to do
with being conservative. The following are examples of
conservative assumptions:

Normal flowrate was used instead of a reduced
estimate

Column tray or overhead flowrate was used instead of
performing a simulation

Multiple unrelated failures occurred simultaneously.

As previously stated, each conservative
assumption should be reviewed and refined so that it is
specific for each system.

Other assumptions. The design and analysis
of relief systems is an art. Much of the analysis is based on
the assumptions that form the overall basis. Mathematical
errors are rarely the cause of an incorrect analysis; usually,
the cause is a problem with the basis. The basis for each
system is stacked on top of a basis for another system. Once
the assumptions are flushed out and determined to be correct,
the mathematics are easy.

Fractionator example. In the past, the
authors reviewed a fractionator (Fig. 1),
where the normal feed vapor rate was specified as the relief
rate for a power failure relief load (conservatively assumed).
When the capacity of the feed furnace was confirmed, the feed
furnace could barely vaporize the normal amount at the normal
production rate and fractionator pressure. This particular
power failure scenario specified the loss of the pumparounds,
which resulted in the loss of approximately 80% of the crude
preheat train duty.

Fig. 1. Flow diagram of an
example fractionator.

With the increase in pressure and cooler-than-normal feed
temperature to feed furnace, the maximum vaporization would be
around 50% of the normal vapor rate. The argument for keeping
the feed preheat was that it was conservative, as the heat
input may not be lost. If this turned out to be the case, then
pumparounds would have continued, leading to a significantly
different outcome. Assumptions need to at least be internally
consistent for each scenario. If the pumparound cooling is
lost, then so is the feed preheat, and vice versa.

Distinguishing minimum compliance from best practices

The final items that need to be reviewed by the plant
engineer are any deviations from RAGAGEP (and not just
deviations from best practices). Often, when completing relief
system projects, the team responsible for the design will, with
the best of intentions, work into guidelines some requirements
that go beyond RAGAGEP.

While extra requirements may be justified based on the
increased safety at nominal incremental costs in new construction, these requirements can
be quite expensive for existing facilities. These additional
requirements must be reviewed and possibly excluded from items
that need to be retrofitted. Regulatory requirements may
require additional documentation to ensure that not making
modifications presents an acceptable risk.2

Gray areas for modification. Often, items
may not be absolutely correct, but they also may not rise to a
level requiring field modification. An example is when current
corporate standards exceed the standards to which a unit was
built. This situation is particularly relevant when a facility
is acquired, thus creating a situation where a facility was
constructed to one set of corporate standards but is now
operating with a new corporate standard in effect.

In these cases, a process designer should investigate any
deviations and document why these deviations are acceptable.
For cases where past designs do not meet the current RAGAGEP
standards, but the deviations are deemed to be minor,
management at some facilities may choose to have more
regulatory risks than safety risks.

Consideration of risk to make changes.
Fixing issues with equipment design, especially when the
facility is running or even in turnaround, must be carried out
with great care. In the past three or four years of literature
searches, the authors have yet to find a single case of a
slightly undersized relief device resulting in an injury or
loss of containment. There are, however, countless records of
injuries sustained from refinery modifications that can be
found via Internet search.

To illustrate this point, in a 2009 Chemical Safety Board
(CSB) video requesting that the city of Houston, Texas adopt
the American Society of Mechanical Engineers (ASME) Pressure
Vessel Code, the CSB was unable to find instances resulting in
loss of containment for pressure vessels for undersized relief
devices.3 The video cites three examples of vessel
failures from undersized relief devices. The first example is a
low-pressure tank with an undersized relief device, and the
other two examples have plugged or isolated vent lines.4,
5, 6

For a plant engineer responsible for increasing overall
facility safety, it may be possible to defer modifications for
the resolution of minor deviations until other equipment
changes are required. This would be at the discretion of the
facility, it would require a reasonable level of risk, and it
could open up the facility to regulatory action.

FLARE SYSTEMS REVIEW METHODOLOGY

The preceding section reviewed the typical methodology that
a process designer would use to generate a relief systems
design basis. This section is designed to help the plant
engineer understand how the individual relief systems loads are
developed and used to create an overall set of global
scenarios, which is then used to verify that the flare system
and associated equipment are adequately designed. Several key
topics will be further explored:

Global load considerations

Reasonable and consistent assumptions

Advanced flare techniques.

By reviewing the flare systems design concern list from
these three angles, a plant engineer can ensure that the basis
for costly changes is justified.

Global load considerations

When a relief systems design project is undertaken, the
individual relief device loads are typically gathered first.
Once these loads are known, they are entered into a hydraulic
analysis tool, and then the flare system is analyzed. However,
as with the individual load determinations, there are areas
that a plant engineer should review.

Credibility of the scenario. In global
scenarios, the process designer typically will review power
failures (both a total loss of power and partial power
failures), utility failures and large-scale liquid pool fires.
All of these scenarios affect multiple systems of equipment and
should be considered. The process designer for each individual
scenario looks at the underlying scenario to ensure that it is
credible. For example:

Is a large-scale liquid pool fire possible, and to what
extent?

Is a total utility failure possible, or does the utility
feed all the listed equipment systems?

Does one utility failure lead to another utility failure
(e.g., loss of steam resulting in the loss of the
turbine-driven instrument air compressor)?

As previously stated, conservative assumptions
for scenarios that are not controlling or that do not have
concerns may be acceptable. A plant engineer should review the
scenario basis for any global scenarios with concerns.
Additionally, the conservative assumptions
associated with the sizing of the relief device may not be
consistent or even possible, given the specific global scenario
being evaluated.

Credibility of the rates. Global
overpressure scenarios are often a compilation of relief rates
specified as closely related individual relief device
scenarios. While these scenarios may have been conservatively
estimated and may have generated no concerns, summing multiple
systems with conservative rates may result in problems.

In a presentation to the 6th Global Congress on Process
Safety, Dustin Smith reported on a refinery-wide review that resulted
in a 40% reduction in the design relief rate by reviewing the
specified relief loads and eliminating overly conservative
assumptions.7 A plant engineer should ensure that
the process designer does not simply create a global scenario
on the basis of multiple conservative calculations; the
designer must also review the system to ensure that rates are
reasonable and defensible (and not excessive due to
assumptions).

Reasonable and consistent assumptions

As with the individual relief systems analysis, the scenario
assumptions and those used to generate the relief rates make a
tremendous impact on the adequacy of the flare system and
associated equipment.

Buried assumptions. When sizing
individual relief devices, RAGAGEP require that the process
designer assumes that the worst case occurs and that all
related failures, pump lineups and control valve responses are
either neutral or detrimental. For global scenarios, the
process designer must assume that the global failure occurs,
but the requirement for neutral or detrimental effects is more
muted. The following are some examples of buried
assumptions typically used:

Heat exchanger duty based on service overall heat
transfer coefficient and area (UA) instead of the clean and
new UA

Level control valves hold level in process vessels

Airfin coolers retain some fractional cooling
capacity

Operations personnel do not simultaneously open
depressuring valves with utility failures unless directed to
in operational procedures.

The plant engineer and process designer should work with
personnel that operate the units, and they should review
scenario basis and loads for any global scenarios with
concerns.

Consistent assumptions. In the definition
of global overpressure scenarios and associated rates, the need
to ensure consistency is paramount. Many times, the process
engineer will assume for one equipment system that a pump was
in operation and has failed, while, in the next equipment
system, the failure pump was spared and the alternative pump
was in operation. For these analyses, consistency across the
facility is required, as the goal is to analyze the flare
system (vs. the individual relief devices). Some assumptions
can result in system-wide inconsistencies:

When a pump is spared and used for multiple equipment
systems, the scenario should specify which pump has failed
for all systems

The effects of the failures must be considered for
systems with heat integration

Utility failures that result in cascading losses must be
examined consistently.

The plant engineer should review the controlling global
scenarios to ensure that the assumptions used are internally
consistent.

Advanced flare analysis techniques

API Standard 521 allows for the consideration of positive
action of instrumentation, operations or other favorable items,
as long as the failure of these items is
considered.8 Prior to making costly flare system
modifications, the plant engineer should review more complex
flare system analysis tools to ensure that modifications are
justified.

Flare load probability analysis. In a
presentation to the 6th Global Congress on Process Safety,
Dustin Smith reported on a method to estimate the flare loading
probability.7 This method determines the likelihood
of loads to the flare system, and it can be used to target
instrumented responses and piping modifications. This method
demonstrates that analysis of the effects of safeguards and the
probability of failure on demand (PFD) of these safeguards can
be used to develop the system loading as a function of
probability/frequency. Using this information and given an
acceptable time frame (e.g., 1 in 100,000 years), the expected
flare load is lower than the worst-case scenario.

The authors recently reviewed a refinery where the likelihood of a
worst-case load, if a total power failure occurred,
was approximately 1 in 100 million years. The design load for 1
in 100,000 years was a fraction of the total load, and it was
more consistent with the complexity of the plant, along with
the DCS programming and the safety instrumented functions and
interlocks recently installed.

Flare quantitative risk assessment. Flare
quantitative risk assessment is a way to review each scenario
and the perturbations of these scenarios to determine the
likelihood of vessel overpressure as a function of
frequency.9 This varies from the flare loading
probability in that the statistical analysis and hydraulic
analysis are coupled; whereas, in the flare loading
probability, the flare loading statistical analysis is separate
from the hydraulic analysis. In both cases, the plant engineer
must ensure that the scenario initiating event frequencies and
the PFD of safeguards are reasonable and defensible.

Flare load dynamic simulations. Offering
and requesting dynamic flare system designs are becoming
increasingly common. Like the other advanced flare analysis
techniques, this one increases the complexity of the analysis,
thus requiring the facility to increase its understanding of
the effects of assumptions on the final answer.10
The basic premise of dynamic simulation is that, by combining
the effects of the staged timing of releases and the dynamic
pressurization of the flare system, the peak loads and back
pressures on system components are reduced. In this method, the
plant engineer must ensure that the fundamental assumptions
affecting the timing of each system or release are reasonable
and defensible, thereby ensuring that the system is properly
modeled.

Other techniques. Other methods to analyze
flare systems are proprietary to operating companies. All of
these methods are designed to account for the probability that
either operator intervention or instrumentation will operate,
or fail to operate, as desired.

Any method of flare header analysis that is not a worst-case
analysis must, therefore, establish some reasonable means of
accounting for the positive action of instrumentation or
operator intervention to mitigate the worst-case load. The
delicate balance between realism and conservativism in flare
header design is paramount in creating a safely designed flare
header at a reasonable cost.11

TAKEAWAY

When reviewing concerns generated from the relief system or
flare design and documentation process, the plant engineer must
ensure that each concern is valid and that any resolution
requiring physical changes is a justified investment of a
facilitys capital. To properly perform this task, it is
recommended that a plant engineer understand how a process
designer performs the study and review the concerns prior to
making physical changes to the facility.

When properly reviewed, upgrades to the flare and relief
system from a relief systems analysis can improve the safety of
an operating facility. HP

Dustin
Smith, PE, is the co-founder and principal
consultant of Smith & Burgess LLC, a process safety
consulting firm based in Houston, Texas. As a consultant,
Mr. Smith has extensive experience with helping
refineries and petrochemicalfacilities maintain compliance
with the PSM standard. He has more than a decade of
experience in relief systems design and PSM compliance.
His experience includes both domestic and international
projects. Mr. Smith is a chemical engineering graduate of
Texas A&M University and a licensed professional
engineer in Texas.

John
Burgess, PE, is the co-founder and principal
consultant of Smith & Burgess LLC, a process safety
consulting firm based in Houston, Texas. Mr. Burgess is a
consultant who specializes in helping refineries and petrochemical plants meet the
PSM standard. His experience includes more than 10 years
in relief systems and PSM compliance, for both domestic
and international projects. Mr. Burgess has BS
and MS degrees in chemical engineering from both Texas
Tech University and the University of Missouri, and he is
a licensed professional engineer in Texas.

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